Alkoxysilanes may be prepared by transesterification of a transesterifiable alkoxysilane (reactant) with an alcohol in the presence of a metal transesterification catalyst to produce an alkoxysilane transesterification reaction product in which at least one alkoxy group of the transesterifiable alkoxysilane is exchanged with the alkoxy group of the esterifying alcohol. As the transesterification reaction proceeds, the byproduct alcohol produced in the transesterification reaction is removed often from the reaction medium to drive the reaction to completion. In the aforedescribed transesterification process, the transesterification catalyst may be separated from the alkoxysilane transesterification reaction product by distillation, or the catalyst may remain in the alkoxysilane transesterification reaction product. Indeed, for some uses, the metal transesterification catalyst presence in the alkoxysilane reaction product may be desirable.
In the aforedescribed process for preparing alkoxysilanes transesterification reaction products, the metal transesterification catalyst, which may be present in the alkoxysilane reaction product, may cause siloxane formation, especially over normal or expected periods of storage, for example, from 1 month to 4 years duration. The siloxanes formation can negatively impact product purity, even to the extent of rendering the product unacceptable for use.
In some cases, retained metal transesterification catalyst may precipitate out of solution thereby interfering with subsequent handling, for example, pumping from storage in the course of a downstream manufacturing operation.
The metal transesterification catalyst may have a boiling point similar to the alkoxysilane reaction product or form an azeotrope with said product. Under these circumstances, the catalyst and product may co-distill and therefore would not effectively remove the metal transesterification catalyst from the alkoxysilane transesterification reaction product. The co-distillation of the catalyst and alkoxysilane transesterification reaction product following completion of the transesterification reaction may also interfere with proper operation of the condenser, specifically, by the accumulation of solids therein. And, as noted above, catalyst present in the alkoxysilane transesterification reaction product may cause siloxane formation or precipitate out of solution.
It will also be noted that metal transesterification catalyst present in the alkoxysilane transesterification reaction product may cause unwanted chemical reaction(s) to occur or accelerate hydrolysis and condensation reaction product of the alkoxysilane transesterification reaction product. These reactions depend on the nature of the alkoxysilane transesterification reaction product and the particular use to which the product is put, as for example, the reaction of epoxides with amines, siloxane formation or crosslinking of silylated polymers, which can reduce the shelf life of an end use product.
A need therefore exists for a process to remove part or substantially all of the metal transesterification catalyst from the alkoxysilane transesterification reaction product, thereby eliminating any interference with the proper operation of condensers during distillation or other separation processes.